Milk fat of humans and other species contains significant quantities of triglycerides with medium chain fatty acids, is readily digestible, and provides the neonate with a major energy source at a time when its lipid-digestive apparatus has not fully matured. The ingestion of milk fat by the neonate induces a mild ketosis and these ketones are apparently important for growth and development. Our program is directed toward elucidating the mechanisms which regulate the production of this important nutrient by the mammary gland. We have shown, with rats, the biosynthesis of medium chain fatty acids requires two enzymes, fatty acid synthetase and thioesterase II. As a result of experiments performed in vivo with normal rats and rats manipulated surgically and hormonally, we have developed the hypothesis that, in the rat mammary gland, increased levels of thioesterase II are associated with the proliferative phase whereas increased levels of fatty acid synthetase are associated with the secretive phase. To test this hypothesis we will be exposed successively to hormonal environments favoring proliferation and then differentiation; the levels of thioesterase II and fatty acid synthetase will be measured. Procedures have been developed for isolation of epithelial cells from human milk; these cells actively synthesize medium chain fatty acids. Factors which may influence the level of thioesterase II, and thus the composition of milk fat, are being evaluated.